1. Field of the Invention
The present invention relates to a voltage generating circuit useful for generating voltages required by, for example, a thin-film-transistor liquid crystal display (TFT-LCD) panel.
2. Description of the Related Art
TFT-LCD panels are used in mobile telephones, to name just one of many applications. The thin-film transistors in a TFT-LCD panel are field-effect transistors through which data signal voltages representing picture element intensity levels or gray levels are applied to capacitors that store charge in proportion to the gray level. The data signal voltages are carried to the source electrodes of the thin-film transistors by source lines (also referred to as data lines) in the TFT-LCD panel.
The data signal voltages are conventionally generated by a resistor ladder and output onto the source lines through a switching circuit that includes a separate voltage-follower amplifier for each source line. A consequent problem is that if the number of source lines is increased to improve the resolution of the display, the number of amplifiers increases proportionally. For a high-resolution display, the numerous amplifiers take up considerable space and consume considerable power.
A second problem is that each amplifier must be capable of generating the full range of output voltages that might be needed on the source line. One known solution to this problem is to use rail-to-rail amplifiers of the push-pull type, but this type of amplifier draws substantial current whenever its output changes, exacerbating the power consumption problem. Another known solution is to use two single-ended amplifiers for each source line, one amplifier operating in the upper half of the output range and the other amplifier operating in the lower half of the output range, and select one amplifier or the other by, for example, comparing the data signal voltage with a reference voltage, but this scheme doubles the number of amplifiers, further increasing the required amount of space, and the comparators or other means that select the amplifiers take up still further space and consume additional power.
The second problem becomes especially troublesome in the alternating-current (ac) driving scheme that is frequently used to improve the response of a TFT-LCD. In one conventional ac driving scheme, the direction of current flow through the resistor ladder is reversed at regular intervals, by reversing the voltages supplied to the two ends of the ladder. Consequently, even when image data values do not change, the amplifiers must deal with frequent large input and output voltage swings, with attendant problems of overshoot, undershoot, and offset. When push-pull amplifiers are used, these large voltage swings are also accompanied by large unwanted transient flows of current through the push-pull output stage.
Another problem with the conventional ac driving scheme is the need to provide switches for switching the voltages supplied to the resistor ladder, and means for controlling the switches.